This invention relates to an optical recording medium on which information can be recorded and/or reproduced using a laser beam. In particular, it relates to a hybrid type high-density optical recording medium which is a recordable recording medium allowing recording with a higher density than a CD and comprises a pre-pit and a pre-groove on a substrate.
Among disk media for optical recording, a recordable compact disk on which recording can be performed only once, and has been popularly used because of its compatibility with a common read-only CD-ROM. For a digital versatile disk (DVD) with a higher recording density than a CD, recordable media have been developed, among which a recordable DVDR is expected to become popular because of its higher compatibility with a DVD-ROM. These recordable media use an organic dye in a recording layer, in which a dye irradiated with a laser beam is locally heated to undergo chemical and/or physical change such as decomposition, deformation, vaporization, melting and solidification to form a pit, allowing information to be recorded.
Meanwhile, there has been proposed an optical recording medium combining a read-only region such as the above CD-ROM and DVD-ROM (ROM region) and a recording region or recordable region such as the above CDR and DVDR (R region) in a single disk. Such specifications will become very important. Such a disk is referred to as a xe2x80x9chybrid diskxe2x80x9d, characterized in that on a substrate, a pre-pit and a pre-groove are formed in a ROM and a R regions, respectively.
Recently, in consumer applications, there has been steady shift from a CD to DVD. In the process, there has been emphasized importance of copyright protection for music, image and moving picture software. Thus, it has become inevitable to thoroughly prevent copying particularly for a DVDR medium permitting recording with the same format. It has been, therefore, standardized to incorporate control data for copy prevention as an ROM region in a data control region on a disk in advance. Specifically, all DVDRs sold in a customer market must be supplied as a hybrid type. There have been needs to propose a convenient and stable hybrid type DVDR as soon as possible.
Although a hybrid disk using an organic dye as a recording film has been standardized for CDR, providing a signal amplitude (recording modulation factor) is more difficult in an ROM region than an R region and therefore, satisfactory properties have not been achieved. It is because a recording layer comprising a dye is formed not only on a pre-groove in the R region but also on a pre-pit in the ROM region so that the same pre-pit shape as that in a commercially available CD does not give an adequate modulation factor.
JP-As 4-146537 and 9-120586 have disclosed that depths of a pre-pit and pre-groove can be adjusted to control a difference in optical path lengths and thus to achieve good signal balance. This technique may, however, have drawbacks that it generally requires difficult stamper cutting of two beams in production, and that a recording signal may be degraded because a relatively shallower groove has a V shape.
There has been proposed an approach that a deposition position of a recording layer is controlled such that the recording layer is formed only over an R region while directly forming a reflective layer over an ROM region. Boundary control is, however, difficult from an R region to an ROM region or an ROM region to an R region. In particular, it cannot be a practical solution in the light of compatibility in reproduction with a CD or DVD player.
A temporary approach has been adopted in most cases, where a pre-pit is formed by in advance conducting recording by a writer in a predetermined region in a recordable disk having only a usual R region for distributing a small number of products. Such an approach cannot be, of course, applied to a mass production.
It is thus desirable for an ROM region in a hybrid disk that the pre-pit information is formed from a stamper template by an appropriate technique such as injection molding.
An objective of this invention is to provide a hybrid disk type of optical recording medium in which a recording layer comprising at least a dye and a reflective layer are sequentially formed on a substrate including an ROM region having a pre-pit and an R region having a pre-groove, exhibiting good recording/reproduction properties meeting DVD specifications in both ROM and R regions.
We have intensely attempted to solve the above problems and have finally found that in a DVDR medium requiring forming a high-density recording pit with a smaller size than a beam spot, the pre-pit and the pre-groove may have the same depth if their depths are more than xcex2/4 and that controlling half value widths of the pre-pit and the pre-groove and providing a tilting angle, in particular a tilting angle in a tangential direction in the pre-pit, allow a signal to be satisfactorily provided from the ROM region. We have also found that a refractive index of a dye material used in a recording film can be adjusted to give more favorable properties, resulting in this invention.
Specifically, this invention provides
1. a recordable optical recording medium comprising a recording layer containing an organic dye which can absorb a laser beam and a metal reflective layer directly or via another layer on a transparent supporting substrate having a pre-groove and a pre-pit, wherein the organic dye has a refractive index nk of 2.2 or more at a reproduction wavelength xcex2; in relation to the reproduction wavelength xcex2, the depths of the pre-groove and the pre-pit on the substrates are more than xcex2/4; and the following equations are satisfied:
0.25rxe2x89xa6wgxe2x89xa60.38r:
0.25xe2x89xa6wp/wgxe2x89xa60.75;
xcex8grxe2x89xa6xcex8pr; and
xe2x80x83xcex8prxe2x89xa6xcex8pt
xe2x80x83wherein r is a recording laser beam diameter represented by xcex1/NA where xcex1 is a recording wavelength [xcexcm] and NA is a numerical aperture for an object lens; wg [xcexcm] and xcex8gr are a half value width and a cross-section tilting angle in a substrate radial direction for the pre-groove, respectively; and wp [xcexcm], xcex8pr and xcex8pt are a half value width, a cross-section tilting angle in a substrate radial direction and a cross-section tilting angle in a tangential direction for the pre-pit, respectively;
2. The recordable optical recording medium as described in paragraph 1 wherein the pre-groove and the pre-pit formed on the substrate further satisfy the relationship of xcex8gr  less than xcex8pt;
3. The recordable optical recording medium as described in paragraph 1 or 2 wherein the pre-groove and the pre-pit have the same depth;
4. The recordable optical recording medium as described in any of paragraphs 1 to 3 wherein the recording layer containing the organic dye is formed on the pre-pit and the pre-groove with a film thickness in the range of 50 nm to 150 nm;
5. The optical recording medium as described in any of paragraphs 1 to 4 wherein the dye contained in the recording layer is an organic dye comprising at least one pyrrole unit or a metal complex thereof;
6. The optical recording medium as described in paragraph 5 wherein the dye contained in the recording layer is a pyrromethene-metal complex represented by formula (1): 
xe2x80x83wherein R1 to R13 independently represent hydrogen, halogen, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C1-C12 alkoxy, and substituted or unsubstituted C6-C20 aryl; and M is a central metal;
7. The optical recording medium as described in any of paragraphs 1 to 6 wherein xcex1 is 0.63 xcexcm to 0.66 xcexcm and NA is 0.60xc2x10.1;
8. The optical recording medium as described in any of paragraphs 1 to 7 having a structure where on a transparent substrate, a recording layer, a reflective layer, an adhesion layer directly or via another layer and another substrate are laminated.